1. Field of the Invention
The present invention relates to radio communication systems, and is more particularly related to identifying a carrier signal in a satellite communication system.
2. Discussion of the Background
Modern satellite communication systems provide a pervasive and reliable infrastructure to distribute voice, data, and video signals for global exchange and broadcast of information. These satellite communication systems have emerged as a viable option to terrestrial communication systems for carrying Internet traffic as well as telephony traffic. Another popular application is direct television broadcasting, which has provided a competitive alternative to cable television systems. Irrespective of the application, a satellite communication system must be designed to support the largest possible traffic capacity, which can translate to lower cost per telephone circuit, or television service, for example, and thus, to increased revenue for the network operator. In addition, satellite communication systems are required to exhibit user response times that are comparable to competing terrestrial systems. That is, the hosts that connect to a satellite terminal (ST) demand that the delay of the satellite system be minimized, thereby reducing user response time.
In recognition of the need to maximize satellite capacity, network operators have deployed various techniques to optimize protocol efficiency. Towards this end, high-speed continuous-mode satellite modems have been deployed; these continuous-mode satellite modems do not use delimiters or the like to separate data frames. For proper operation of the satellite system, it is necessary to distinguish a desired carrier from n identical adjacent carriers in the presence of frequency uncertainties at the receiver. These frequency uncertainties introduce frequency spacings that are greater than the spacing between the desired and adjacent carriers. Many factors contribute to the frequency uncertainties in satellite communication systems, such as the transmitter oscillator, satellite transponder, low-noise-block converter (LNB), and the receiver tuner. The frequency uncertainty could be quite significant in the down-converted signal, especially at low data rates. As a result, a satellite modem may demodulate the wrong carrier. For burst-mode communication, different unique words (UW) in the burst preamble can be applied to identify different carriers. However in continuous-mode communication, use of such unique words presents undesirable overhead, thereby requiring additional bandwidth beyond that necessary to transmit the actual data. The extra transmission bandwidth requirement is prohibited under certain scenarios, in which the bandwidth of each user is pre-allocated. Thus, it is desirable to provide carrier identification functionality without consuming precious additional bandwidth sacrificing performance (bit error rate (BER) and acquisition time), or adding delay uncertainty in the receiver demodulation-decoding chain. Such delay uncertainty in the receiver may cause synchronization problems in network.
One conventional approach to carrier identification employs a small interleaver (i.e., shuffler) between the convolutional encoder and a shaping filter in the transmitter, in which a deinterleaver is applied before a Viterbi decoder in the receiver, using a synchronization flag (denoted as xe2x80x9csyncxe2x80x9d) from the Viterbi decoder to adjust the de-interleaving state. A different interleaving scheme is derived for different adjacent carrier. The drawbacks with this approach are that the complexity of the hardware is increased and the de-interleaving delay is unpredictable.
Based on the foregoing, there is a clear need for improved approaches to distinguishing a wanted carrier from multiple adjacent carriers in the presence of frequency uncertainties within the receiver of a radio communication system.
There is also a need to minimize hardware modifications of presently available modems.
There is also a need to detect the wanted carrier without utilizing overhead bits, thereby not requiring additional system bandwidth.
There is also a need to avoid the introduction of delay in the demodulation and decoding process.
There is also a further need to minimize additional acquisition time during the modem acquisition operation.
Therefore, an approach for performing carrier identification using a more efficient algorithm is highly desirable.
According to one aspect of the invention, a method is provided for identifying a particular carrier among a plurality of carriers. The method includes multiplying an encoded signal by a prescribed periodic sequence to form a watermarked signal. In addition, the method encompasses transmitting the watermarked signal associated with the particular carrier over a communication channel. The method further includes tuning to one of the carriers, extracting a data sequence from the one carrier, multiplying the data sequence with the prescribed periodic sequence to form another watermarked signal, and determining whether the watermarked signals match. This approach advantageously minimizes hardware modifications to legacy modems.
According to another aspect of the invention, a transmitting device for generating a plurality of carriers comprises a forward error control (FEC) encoder that is configured to code data bits from a data terminal equipment (DTE) to output an encoded signal. A puncturing/carrier identification encoding logic is configured to multiply the encoded signal by a prescribed periodic sequence to form a watermarked signal, to puncture the encoded signal according to a puncturing scheme, and to synchronize the prescribed periodic sequence with the puncturing scheme. A modulator is configured to modulate the watermarked signal. The above arrangement advantageously provides support of multiple carriers without requiring additional system bandwidth.
According to another aspect of the invention, a receiving device for receiving a plurality of carriers comprises a tuner that is configured to tune to one of the plurality of carriers. A demodulator is configured to extract a data sequence from the one carrier. A carrier identification (ID) module is configured to multiply the data sequence with a prescribed periodic sequence to form a watermarked signal. The carrier ID module provides a feedback signal to the tuner to control selection of the plurality of carriers. A forward error control (FEC) decoder is configured to decode selectively the watermarked signal. The above arrangement advantageously avoids the introduction of delay in the demodulation and decoding process.
According to another aspect of the invention, a communication system utilizing a plurality of carriers in a continuous-mode modem comprises a transmitter that is configured to multiply an encoded signal by a prescribed periodic sequence to form a watermarked signal and to transmit the watermarked signal associated with a particular carrier over a communication channel. A receiver is configured to tune to one of the carriers and to extract a data sequence from the one carrier. The receiver multiplies the data sequence with the prescribed periodic sequence to form another watermarked signal and determines whether the watermarked signals match. The above arrangement advantageously performs carrier identification efficiently.
According to yet another aspect of the invention, a system for identifying a particular carrier among a plurality of carriers comprises means for multiplying an encoded signal by a prescribed periodic sequence to form a watermarked signal. The system also includes means for transmitting the watermarked signal associated with the particular carrier over a communication channel, means for tuning to one of the carriers, means for extracting a data sequence from the one carrier, means for multiplying the data sequence with the prescribed periodic sequence to form another watermarked signal, and means for determining whether the watermarked signals match. Under the above approach, acquisition time during modem acquisition operation is minimized.
In yet another aspect of the invention, a computer-readable medium carrying one or more sequences of one or more instructions for receiving a plurality of carriers is disclosed. The one or more sequences of one or more instructions include instructions which, when executed by one or more processors, cause the one or more processors to perform the step of tuning to one of the plurality of carriers. Another step includes extracting a data sequence from the one carrier. Another step includes multiplying the data sequence with a prescribed periodic sequence to form a watermarked signal. Yet another step includes selectively decoding the watermarked signal. This approach advantageously avoids use of overhead bits to identify carriers.